yoda: warning cleanup sweep (dead code + clippy --fix) (squashed)

Squash of 4 yoda commits:
- 84437e21 yoda: libcosmic-yoda dead-code purge (14->0 warnings)
- 999db0a4 yoda: cosmic-theme cleanup (4->0 warnings) — workspace at 0 warnings total
- 4743bb8e yoda: cargo clippy --fix on libcosmic-yoda (115->33 warnings)
- 675f3b59 chore: reduce local stack warnings

src/core.rs

@@ -92,8 +92,6 @@ pub struct Core {
 
     pub(super) portal_accent: Option<Srgba>,
 
-    pub(super) portal_is_high_contrast: Option<bool>,
-
     pub(super) title: HashMap<Id, String>,
 
     pub window: Window,
@@ -183,7 +181,6 @@ impl Default for Core {
             settings_daemon: None,
             portal_is_dark: None,
             portal_accent: None,
-            portal_is_high_contrast: None,
             main_window: None,
             exit_on_main_window_closed: true,
             menu_bars: HashMap::new(),